Environmental Engineering Reference
In-Depth Information
Dam owners, dam designers and all others involved in the implementation of a large
dam project ignore the geological/geotechnical matters of their damsite and dam at their
peril. All, including the geological/geotechnical team members, must ask the awkward
question about potential deep-seated instability early in the process. One can only imagine
the disaster at Thomson if the lingering doubts of the dam designer in 1977 about the “dis-
turbed rock and potential instability at the damsite” had been lost in the desire to get the
“job moving” and, for whatever reason or luck, the 350,000 m
3
of the right abutment that
did move did not do so. That the project was set back by 5 years or so was a major concern
to the owner and the designer, but that concern could be nothing compared to a major
failure during initial reservoir filling, which is very likely what would have happened had
the major stabilising fill works not been constructed.
2.11
STABILITY OF SLOPES AROUND STORAGES
The slopes around a proposed storage area may include some which are intact and stable,
others intact but less stable, others formed by dormant landslides, and still others formed
by active landslides. The local raising of the water table which occurs when the storage is
filled, and lowering when the storage level is drawn down, may reduce the stability of any
of these slopes. Sometimes the reduced stability causes landsliding in a previously intact
slope. This landslide is called a “first-time” slide (see
2.10.1
). For a dormant landslide, or
an active or occasionally active slide, the reduced stability can cause reactivation of the
former, or increased movement rates in the latter. Affected slopes are normally within the
storage area (Figures 2.41 and
2.42
). However, slope failure is also possible (but rare) on
the outer side of a storage rim (
Figure 2.43
).
The consequences of landsliding within the storage area can include
-
Damage to existing property or infrastructure;
-
Constraints on operation of the storage;
-
Damage to roads;
-
Damage to the dam, or its foundations;
-
Damage to or blocking of intake or outlet works;
Mass with potential to slide
(a)
Water table before
filling the storage
Mass with potential to slide
Full supply level
(b)
Minimum operating level
Water table after
filling the storage
Figure 2.41.
Effect of filling and operating a storage, on the stability of a jointed rock slope partly
inundated by it. (a) Before filling the storage (b) During operation.
